Silver amalgams have been used successfully for decades as filling material for teeth damaged by tooth diseases. They are especially useful for this purpose because of their high mechanical strength, long life and ease of use. The amalgam is produced at the dentist's office from a suitable alloyed powder and pure mercury. The alloyed powders normally consist of silver, copper and tin and frequently also of a little zinc. The silver content varies from 24 to 93% by weight (cf. for example published German Patent Specification DE-OS No. 25 11 194 and U.S. Pat. Nos. 3,985,885, 4,039,329, 3,975,192, 4,030,918, 3,762,917 and 3,871,876). Other additives are also used in some cases, such as indium (see e.g. U.S. Pat. No. 4,030,918) or palladium (U.S. Pat. No. 4,374,085).
The alloyed powders are normally produced using molten metallurgical techniques, either by spraying the melt or by machining a cast molded body (e.g. a bar or a billet).
When the spraying process is used, the powder is produced directly from the melt by passing the melt stream through a funnel and then through an annular nozzle loaded with water or gas, after which the stream is atomized. As a result of the high cooling speed which occurs in this process, the particles which are formed are primarily spherical. Also, the alloy phases, e.g. Ag.sub.3 Sn and Cu.sub.3 Sn, are present in a finely distributed fashion. Amalgams with great strength are obtained from such powders, especially in the case of low silver contents. On account of the spherical shape of the powder particles and the associated high bulk density, the amount of mercury required for producing a pasty tamping body is less than in the case of chip amalgams produced by machining; however, the working properties are not optimum since the individual particles slide past each other too easily during tamping and thus escape the tamping pressure. Moreover, the working properties of such so-called "ball amalgams" react in a considerably more sensitive manner to errors in measuring the proportions of alloy particles and mercury than do chip amalgams. Further, the manufacturing cost of the sprayed alloy is high, largely because of the low yield of utilizable powder.
On the other hand, when the alloying powder is made by machining, there are increases and a coarse distribution of the Ag.sub.3 Sn and Cu.sub.3 Sn phases in the molded body and in the alloyed powder. These arise because of the relatively low cooling speed during the production of the cast bar which is machined. This characteristic has a negative effect on the binding reaction with mercury. The technical properties such as e.g. binding expansion and strength are also adversely affected. In addition, the yield of utilizable powder at 30-50% is very low so that increased costs result due to subsequent grinding processes and any separated waste which occurs. An advantage of the chip amalgams over the ball amalgams is their very good workability.
Published German Patent Specification DE-PS No. 32 40 356 describes a method of producing an amalgam powder which uses sintering technology. First, a sprayed alloyed powder is produced from silver-tin-copper which is subsequently compacted and sintered. The porous, sintered, alloyed molded body is then machined. In this manner, an improvement in the quality of the amalgam is achieved. However, this method has the disadvantage of high production costs, which arise chiefly as a result of the production of the alloyed powder via spraying.